How Vegetative Propagation Works
Many plants propagate themselves vegetatively by means of crowns,, suckers, rhizomes, tubers, natural , and of course by bulbs and corms. We are concerned in this section, not with natural but with ‘artificial’ methods of vegetative propagation, primarily the use of stem cuttings.
Inducing the sections of vegetative tissue taken from a plant to form roots is a technique with many advantages over seed propagation. Speed is one of them, so also is the production of a plant with identical characteristics to the parent (with certain exceptions). Obviously this has great advantages in propagation, as it enables good forms to be readily perpetuated. Coleus blumei, for example, is readily raised from seed and the resulting seedlings are generally varied and in many cases of interesting colour form. To sow seed selected from a desirable openly pollinated plant would most likely result in further variance, whereas cuttings taken and rooted will perpetuate the form and colour: These are commonly called ‘clones’.
The main disadvantage of vegetative propagation concerns possible disease transmission. Some degree of selection must of course be constantly exercised by a vigorous process of roguing, as is necessary with dahlias propagated from cuttings arising from over-wintered tubers. While the F1 seedlings of dahlias offer endless possibilities for bedding display, cuttings arc the only simple way of ensuring the fixed forms and disease transmission is therefore a major problem in both commercial and amateur circles. This is also true of. The use of heat therapy to “burn out” virus infection and so enable only clean stock to be vegetatively propagated can be found in specialist books, and is possible a technique which amateurs may hesitate to use.
Botanical aspects of vegetative propagation
The ability of a plant to ‘think’ and pursue a deliberate course of planned action is still a matter of dispute in the botanical world. That a plant is without feelings and merely accepts its fate with stoicism is not accepted by some botanists, the most recent work in this direction being done in Russia. The fact remains, however, that the gardener, not the plant, must generally make the decision when and how to propagate, and at the same time try to interpret the optimum conditions for encouraging the portion of plant selected to become self-supporting as soon as possible. ‘Wound shock’ invariably arises when it is severed from its parent; it must be encouraged over the shock, and simultaneously persuaded to put out roots.
The first stage of propagation is when the cambium layer which exists between the phloem and the xylem in dicotyledonous plants forms a skin or callus over the wound. This is both to prevent loss of moisture and to protect against fungal and bacterial invasion. This cambium layer then stimulates either dormant buds or root primordia to form, both in the area of severance and further up the stem. It is interesting to note that while buds form stems above ground from external tissue, roots are formed from internal tissue. There must obviously be a subtle differentiation in root origin between different types and portions of plants. If, for example, you take a cutting from a chrysanthemum stool orwell down near the root (‘Irishman’s cuttings’) when there will no doubt be root initials present, then roots will develop externally. What significance this may have on the ultimate performance of the newly formed plant is a matter for hot debate in specialist exhibition circles, yet for the average gardener the theory of it all matters little, the important issue being the formation of roots. It must also be repeated that the role of the cambium is vital in most aspects of propagation (including grafting and budding) to bring about union between two sections of a plant.
Hygiene is important when removing a portion of plant from its parent, and so is the avoidance of jagged tears, so leaving tissue without a cambium layer to form a skin and protect itself against attack from weak pathogenic organisms. On the other hand, there is an increasing use of the snap-off technique of severing cuttings (avoiding any mechanical means other than those necessary to remove the stipules or leaves), this being practised almost universally in the nurseries of specialist chrysanthemum and pelargonium raisers. This is perhaps mainly a preventive measure against the spreading of disease, though it is also contended that the tissue is more severely wounded by mechanical means, whether it be with a knife or a razor blade, cells actually being dissected, whereas with snapping the cells break off at the cell walls. This raises the further interesting line of thought that the urge to recover from the shock of snapping off might be less than that experienced on mechanical removal.
Hormones or auxins
Hormones or auxins occur naturally in all living plants and are responsible for triggering off cell activity or chemical processes. They are chemical ‘messengers’ which encourage changes in activity without themselves becoming changed in nature. Their effect on growth behaviour is profound and they are conveyed rapidly around the plant according to the special demands necessitated by changing circumstances, of which the initiation of new roots on plant sections removed from the parent is a typical example.
Should there be a shortage of hormones or auxins in the portion of plant removed from the parent, research work has shown that certain chemical substances will produce a response similar to that produced by natural hormones, and indeed may in fact be more effective than natural hormones in certain instances. Such materials are called growth regulators and are used for a wide range of purposes including weed control, growth suppression and, most important in the context of propagation, inducing new roots to form. How these hormones act on plants is complicated in the extreme, but obtaining the right response is largely a matter of using the correct material and the proper dilution at the right time.
While, generally speaking, the use of hormones speeds up the rooting process, this is not always the case, nor is it certain that the hormones will induce roots to form on difficult species. Experience over the years shows that an artificial hormone material is more useful on soft leafy material and at fairly high propagating temperatures in the 18-24°C (65-75°F) range. Solutions of the hormone can be used to immerse the base of the prepared cutting material for 18-24 hours; it is rinsed with water before insertion. The more popular method is to dip the wet base of the prepared cutting in a powder containing the hormone and this is perhaps more hygienic. Directions on the use of hormones are issued with the respective products and should be closely followed.